Aluminum and aluminum alloy ingots and billets are routinely formed by the direct chill (DC) casting process. In this process, molten metal is fed to a mold having a bottom portion thereof which is lowered as the metal solidifies, forming the ingot or billet from a molten pool of metal at the top of the mold. Solidification of the molten metal results from water spray cooling of the mold and the outer surface of the metal ingot or billet.
The DC casting method is not without its limits. Surface cracks and other surface imperfections require that a significant portion of the outer surface of the ingot or billet be removed or scalped from the ingot or billet after casting and prior to rolling or extrusion into a final product.
With certain aluminum alloys, notably the relatively hard aluminum can body stock alloys, such as aluminum alloy 5182, these surface irregularities are often pronounced. It is evident from close examination of finished ingots of this material that a major cause of the surface irregularities comes from the metal solidifying in a series of horizontal layers, rather than in a continuous vertical freeze pattern. This requires even more extensive scalping of the ingots, resulting in excessive scrap.
Another common surface characteristic of ingots formed from alloys such as aluminum alloy 5182 is a darkened surface appearance, referred to as a black ingot.
Other surface defects in aluminum ingots and billets as they are formed by the DC casting method include tearing of the surface resulting from sticking of the surface of the ingot to th-e surface of the mold.
In the past, various lubrication schemes have been attempted to improve aluminum ingot and billet casting surface quality. Such methods as coating the mold surface with a release agent prior to casting, supplying a soluable oil to the mold during casting and applying carbon powder to the molter metal surface of the ingot during casting have met with mixed results.
It is desirable, therefore, to provide an improved method for direct chill casting of aluminum and aluminum alloy ingots and billets which will substantially reduce or eliminate surface cracks, tearing, and other surface defects, such as black ingots, so that reduced levels of scalping prior to final forming are required.